QTL Pyramiding and Its Use in Breeding for Increasing the Phytoextraction Efficiency of Soil Cd via High-Cd-Accumulating Rice.

Phytoextraction by high-Cd-accumulating rice lacking a functional OsHMA3 allele is promising for Cd removal from paddy soils. To increase rice Cd extraction efficiency, we developed a new high-Cd variety, TJN25-11. For this, we pyramided a nonfunctional OsHMA3 allele from a high-Cd variety, Jarjan, and two QTLs for increased shoot Cd concentrations, which were discovered in a mapping population derived from a high-Cd variety, Nepal 555, and a low-Cd variety, Tachisugata. In two Cd-contaminated paddy fields under drained aerobic soil conditions, TJN25-11 presented significantly higher Cd concentrations in the straw and panicles than the OsHMA3-deficient varieties TJTT8 and Cho-ko-koku. Among the varieties, TJN25-11 had a relatively high shoot biomass, resulting in the highest Cd accumulation in the shoots. The soil Cd decreased by approximately 20% after TJN25-11 growth. The amount of Cd that accumulated in the TJN25-11 aerial parts was much greater than the amount of Cd that decreased in the topsoil, suggesting that Cd was absorbed from deeper soil layers. Thus, we revealed the effects of QTL pyramiding on shoot Cd accumulation and Cd phytoextraction efficiency. Since TJN25-11 has favorable agronomic traits for compatibility with Japanese cultivation systems, this variety could be useful for Cd phytoextraction in Cd-contaminated paddy fields.

A weak allele of OsNRAMP5 confers moderate cadmium uptake while avoiding manganese deficiency in rice.

Decreasing cadmium (Cd) concentrations in rice grains can effectively reduce potential risks to human health because rice is the major contributor to Cd intake in many diets. Among several genes involved in rice Cd accumulation, the loss of function of OsNRAMP5 is known to be effective in reducing grain concentration by inhibiting root uptake. However, disruption of this gene simultaneously decreases manganese (Mn) uptake because OsNRAMP5 is a major Mn transporter. With the aim of improving Mn uptake in OsNRAMP5 mutants while still restricting the grain Cd concentration below the upper limit of international standards, we identified a novel OsNRAMP5 allele encoding a protein in which glutamine (Q) at position 337 was replaced by lysine (K). The mutant carrying the OsNRAMP5-Q337K allele showed intermediate Cd and Mn accumulation between that of the wild-type and OsNRAMP5-knockout lines, and exhibited more resistance to Mn deficiency than the knockout lines. Different amino acid substitutions at position Q337 significantly affected the Cd and Mn transport activity in yeast cells, indicating that it is one of the crucial sites for OsNRAMP5 function. Our results suggest that the OsNRAMP5-Q337K allele might be useful for reducing grain Cd concentrations without causing severe Mn deficiency in rice cultivars through DNA marker-assisted breeding.

Tonoplast-Localized OsMOT1;2 Participates in Interorgan Molybdate Distribution in Rice.

Molybdenum (Mo) is an essential element for plant growth and is utilized by several key enzymes in biological redox processes. Rice assimilates molybdate ions via OsMOT1;1, a transporter with a high affinity for molybdate. However, other systems involved in the molecular transport of molybdate in rice remain unclear. Here, we characterized OsMOT1;2, which shares amino acid sequence similarity with AtMOT1;2 and functions in vacuolar molybdate export. We isolated a rice mutant harboring a complete deletion of OsMOT1;2. This mutant exhibited a significantly lower grain Mo concentration than the wild type (WT), but its growth was not inhibited. The Mo concentration in grains was restored by the introduction of WT OsMOT1;2. The OsMOT1;2-GFP protein was localized to the vacuolar membrane when transiently expressed in rice protoplasts. At the reproductive growth stage of the WT plant, OsMOT1;2 was highly expressed in the 2nd and lower leaf blades and nodes. The deletion of OsMOT1;2 impaired interorgan Mo allocation in aerial parts: relative to the WT, the mutant exhibited decreased Mo levels in the 1st and 2nd leaf blades and grains but increased Mo levels in the 2nd and lower leaf sheaths, nodes and internodes. When the seedlings were exposed to a solution with a high KNO3 concentration in the absence of Mo, the mutant exhibited significantly lower nitrate reductase activity in the shoots than the WT. Our results suggest that OsMOT1;2 plays an essential role in interorgan Mo distribution and molybdoenzyme activity in rice.

Deficiency in alcohol dehydrogenase 2 reduces arsenic in rice grains by suppressing silicate transporters.

Paddy fields are anaerobic and facilitate arsenite (As(III)) elution from the soil. Paddy-field rice accumulates arsenic (As) in its grains because silicate transporters actively assimilate As(III) during the reproductive stage. Reducing the As level in rice grains is an important challenge for agriculture. Using a forward genetic approach, we isolated a rice (Oryza sativa) mutant, low arsenic line 3 (las3), whose As levels were decreased in aerial tissues, including grains. The low-As phenotype was not observed in young plants before heading (emergence of the panicle). Genetic analyses revealed that a deficiency in alcohol dehydrogenase (ADH) 2 by mutation is responsible for the phenotype. Among the three rice ADH paralogues, ADH2 was the most efficiently produced in root tissue under anaerobic conditions. In wild-type (WT), silicon and As concentrations in aerial tissues increased with growth. However, the increase was suppressed in las3 during the reproductive stage. Accordingly, the gene expression of two silicate transporters, Lsi1 and Lsi2, was increased in WT around the time of heading, whereas the increase was suppressed in las3. These results indicate that the low-As phenotype in las3 is due to silicate transporter suppression. Measurement of intracellular pH by 31P-nuclear magnetic resonance revealed intracellular acidification of las3 roots under hypoxia, suggesting that silicate transporter suppression in las3 might arise from an intracellular pH decrease, which is known to be facilitated by a deficiency in ADH activity under anaerobic conditions. This study provides valuable insight into reducing As levels in rice grains.

Domain exchange between Oryza sativa phytochelatin synthases reveals a region that determines responsiveness to arsenic and heavy metals.

Phytochelatin synthases (PCSs) are activated by toxic metals/metalloids such as cadmium and arsenic and synthesize phytochelatins for detoxification of toxic elements. Rice (Oryza sativa L.) has two PCSs (OsPCS1 and OsPCS2), and we previously revealed that OsPCS1 has a higher responsiveness to arsenic than to cadmium, while OsPCS2 has a higher responsiveness to cadmium than to arsenic. Moreover, we found that the specific responsiveness of OsPCS1 to arsenic at rice nodes is a key factor in reducing arsenic in rice grains. However, the molecular characteristics of two PCSs in rice that contribute to the responsiveness to arsenic or heavy metals, including Cd, remain unclear. Here, we experimentally demonstrate that the C-terminal region in PCSs determines the responsiveness to arsenic or cadmium. We constructed chimeric proteins between OsPCS1 and OsPCS2 and performed an in vitro phytochelatin synthesis assay. A chimeric protein in which the 183 C-terminal amino acids of OsPCS2 were replaced with the 185 C-terminal amino acids of OsPCS1 showed higher responsiveness to arsenite than to cadmium, similar to OsPCS1. Contrary to expectations, mutations of cysteine residues that are unique to OsPCS1 or OsPCS2 had little influence on the responsiveness, although cysteine residues are reported to be representative of sites that interact with metals/metalloids. These results would enable the development of a breeding technology for reducing arsenic in rice grains by improving the arsenic-dependent activation of PCSs.

Arsinothricin, an arsenic-containing non-proteinogenic amino acid analog of glutamate, is a broad-spectrum antibiotic.

The emergence and spread of antimicrobial resistance highlights the urgent need for new antibiotics. Organoarsenicals have been used as antimicrobials since Paul Ehrlich's salvarsan. Recently a soil bacterium was shown to produce the organoarsenical arsinothricin. We demonstrate that arsinothricin, a non-proteinogenic analog of glutamate that inhibits glutamine synthetase, is an effective broad-spectrum antibiotic against both Gram-positive and Gram-negative bacteria, suggesting that bacteria have evolved the ability to utilize the pervasive environmental toxic metalloid arsenic to produce a potent antimicrobial. With every new antibiotic, resistance inevitably arises. The arsN1 gene, widely distributed in bacterial arsenic resistance (ars) operons, selectively confers resistance to arsinothricin by acetylation of the α-amino group. Crystal structures of ArsN1 N-acetyltransferase, with or without arsinothricin, shed light on the mechanism of its substrate selectivity. These findings have the potential for development of a new class of organoarsenical antimicrobials and ArsN1 inhibitors.

Correction to: Genetic diversity of arsenic accumulation in rice and QTL analysis of methylated arsenic in rice grains.

The authors of article "Genetic diversity of arsenic accumulation in rice and QTL analysis of methylated arsenic in rice grains" (Kuramata et al. 2013) would like to note that the original version of the article online unfortunately contains the following errors.

Phytochelatin synthase OsPCS1 plays a crucial role in reducing arsenic levels in rice grains.

Reduction of the level of arsenic (As) in rice grains is an important challenge for agriculture. A recent study reported that the OsABCC1 transporter prevents the accumulation of As in grains by sequestering As-phytochelatin complexes into vacuoles in the upper nodes. However, how phytochelatins are provided in response to As remains unclear. Here, we show that the phytochelatin synthase OsPCS1 plays a crucial role in reducing As levels in rice grains. Using a forward genetic approach, we isolated two rice mutants (has1 and has2) in which As levels were much higher in grains but significantly lower in node I compared with the wild type. Map-based cloning identified the genes responsible as OsABCC1 in has1 and OsPCS1 in has2. The levels of As in grains and node I were similar between the two mutants, suggesting that OsABCC1 preferentially cooperates with OsPCS1 to sequester As, although rice has another phytochelatin synthase, OsPCS2. An in vitro phytochelatin synthesis assay indicated that OsPCS1 was more sensitive to activation by As than by cadmium, whereas OsPCS2 was more weakly activated by As than by cadmium. Transgenic plants highly expressing OsPCS1 showed significantly lower As levels in grains than did wild-type plants. Our results provide new knowledge of the relative contribution of rice PCS paralogs to As sequestration and suggest a good candidate tool to reduce As levels in rice grains.

Low-cesium rice: mutation in OsSOS2 reduces radiocesium in rice grains.

In Japan, radiocesium contamination in foods has become of great concern and it is a primary issue to reduce grain radiocesium concentration in rice (Oryza sativa L.). Here, we report a low-cesium rice mutant 1 (lcs1) with the radiocesium concentration in grain about half that in the wild-type cultivar. Genetic analyses revealed that a mutation in OsSOS2, which encodes a serine/threonine-protein kinase required for the salt overly sensitive (SOS) pathway in plants, is responsible for the decreased cesium (Cs) concentrations in lcs1. Physiological analyses showed that Cs+ uptake by lcs1 roots was significantly decreased under low-potassium (K+) conditions in the presence of sodium (Na+) (low K+/Na+). The transcript levels of several K+ and Na+ transporter genes, such as OsHAK1, OsHAK5, OsAKT1, and OsHKT2;1 were significantly down-regulated in lcs1 grown at low K+/Na+. The decreased Cs+ uptake in lcs1 might be closely related to the lower expression of these genes due to the K+/Na+ imbalance in the lcs1 roots caused by the OsSOS2 mutation. Since the lcs1 plant had no significant negative effects on agronomic traits when grown in radiocesium-contaminated paddy fields, this mutant could be used directly in agriculture for reducing radiocesium in rice grains.

Detoxification of hydroxylated polychlorobiphenyls by Sphingomonas sp. strain N-9 isolated from forest soil.

To examine the biodegradation of hydroxylated polychlorobiphenyls (OH-PCBs), we isolated Sphingomonas sp. strain N-9 from forest soil using mineral salt medium containing 4-hydroxy-3-chlorobiphenyl (4OH-3CB) at the concentration of 10 mg/L. Following incubation with strain N-9, the concentration of 4OH-3CB decreased in inverse proportion to strain N-9 proliferation, and it was converted to 3-chloro-4-hydroxybenzoic acid (4OH-3CBA) after 1 day. We observed that strain N-9 efficiently degraded lowly chlorinated OH-PCBs (1-4 Cl), while highly chlorinated OH-PCBs (5-6 Cl) were less efficiently transformed. Additionally, strain N-9 degraded PCBs and OH-PCBs with similar efficiencies, and the efficiency of OH-PCB degradation was dependent upon the positional relationships between OH-PCB hydroxyl groups and chlorinated rings. OH-PCB biodegradation may result in highly toxic products, therefore, we evaluated the cytotoxicity of two OH-PCBs [4OH-3CB and 4-hydroxy-3,5-dichlorobiphenyl (4OH-3,5CB)] and their metabolites [4OH-3CBA and 3,5-chloro-4-hydroxybenzoic acid (4OH-3,5CBA)] using PC12 rat pheochromocytoma cells. Our results revealed that both OH-PCBs induced cell membrane damage and caused neuron-like elongations in a dose-dependent manner, while similar results were not observed for their metabolites. These results indicated that strain N-9 can convert OH-PCBs into chloro-hydroxybenzoic acids having lower toxicity.

Arsenic biotransformation by Streptomyces†sp. isolated from rice rhizosphere.

Isolation and functional analysis of microbes mediating the methylation of arsenic (As) in paddy soils is important for understanding the origin of dimethylarsinic acid (DMA) in rice grains. Here, we isolated from the rice rhizosphere a unique bacterium responsible for As methylation. Strain GSRB54, which was isolated from the roots of rice plants grown in As-contaminated paddy soil under anaerobic conditions, was classified into the genus Streptomyces by 16S ribosomal RNA sequencing. Sequence analysis of the arsenite S-adenosylmethionine methyltransferase (arsM) gene revealed that GSRB54 arsM was phylogenetically different from known arsM genes in other bacteria. This strain produced DMA and monomethylarsonic acid when cultured in liquid medium containing arsenite [As(III)]. Heterologous expression of GSRB54 arsM in Escherichia coli promoted methylation of As(III) by converting it into DMA and trimethylarsine oxide. These results demonstrate that strain GSRB54 has a strong ability to methylate As. In addition, DMA was detected in the shoots of rice grown in liquid medium inoculated with GSRB54 and containing As(III). Since Streptomyces are generally aerobic bacteria, we speculate that strain GSRB54 inhabits the oxidative zone around roots of paddy rice and is associated with DMA accumulation in rice grains through As methylation in the rice rhizosphere.

Detection of QTLs to reduce cadmium content in rice grains using LAC23/Koshihikari chromosome segment substitution lines.

To advance the identification of quantitative trait loci (QTLs) to reduce Cd content in rice (Oryza sativa L.) grains and breed low-Cd cultivars, we developed a novel population consisting of 46 chromosome segment substitution lines (CSSLs) in which donor segments of LAC23, a cultivar reported to have a low grain Cd content, were substituted into the Koshihikari genetic background. The parental cultivars and 32 CSSLs (the minimum set required for whole-genome coverage) were grown in two fields with different natural levels of soil Cd. QTL mapping by single-marker analysis using ANOVA indicated that eight chromosomal regions were associated with grain Cd content and detected a major QTL (qlGCd3) with a high F-test value in both fields (F = 9.19 and 5.60) on the long arm of chromosome 3. The LAC23 allele at qlGCd3 was associated with reduced grain Cd levels and appeared to reduce Cd transport from the shoots to the grains. Fine substitution mapping delimited qlGCd3 to a 3.5-Mbp region. Our results suggest that the low-Cd trait of LAC23 is controlled by multiple QTLs, and qlGCd3 is a promising candidate QTL to reduce the Cd level of rice grain.

Genetic diversity of arsenic accumulation in rice and QTL analysis of methylated arsenic in rice grains.

BACKGROUND: Rice is a major source of dietary intake of arsenic (As) for the populations that consume rice as a staple food. Therefore, it is necessary to reduce the As concentration in rice to avoid the potential risk to human health. In this study, the genetic diversity in As accumulation and As speciation in rice grains was investigated using a world rice core collection (WRC) comprising 69 accessions grown over a 3-year period. Moreover, quantitative trait locus (QTL) analysis was conducted to identify QTLs controlling the dimethylarsinic acid (DMA) content of rice grains. RESULTS: There was a 3-fold difference in the grain As concentration of WRC. Concentrations of total-As, inorganic As, and DMA were significantly affected by genotype, year, and genotype-year interaction effects. Among the WRC accessions, Local Basmati and Tima (indica type) were identified as cultivars with the lowest stable total-As and inorganic As concentrations. Using an F2 population derived from Padi Perak (a high-DMA accession) and Koshihikari (a low-DMA cultivar), we identified two QTLs on chromosome 6 (qDMAs6.1 and qDMAs6.2) and one QTL on chromosome 8 (qDMAs8) that were responsible for variations in the grain DMA concentration. Approximately 73% of total phenotypic variance in DMA was explained by the three QTLs. CONCLUSIONS: Based on the results provided, one strategy for developing rice cultivars with a low level of toxic As would be to change the proportion of organic As on the basis of a low level of total As content.

Rice DEP1, encoding a highly cysteine-rich G protein γ subunit, confers cadmium tolerance on yeast cells and plants.

A rice cDNA, OsDEP1, encoding a highly cysteine (Cys)-rich G protein γ subunit, was initially identified as it conferred cadmium (Cd) tolerance on yeast cells. Of the 426 aa constituting OsDEP1, 120 are Cys residues (28.2%), of which 88 are clustered in the C-terminal half region (aa 170-426). To evaluate the independent effects of these two regions, two truncated versions of the OsDEP1-expressing plasmids pOsDEP1(1-169) and pOsDEP1(170-426) were used to examine their effects on yeast Cd tolerance. Although OsDEP1(170-426) conferred a similar level of Cd tolerance as the intact OsDEP1, OsDEP1(1-169) provided no such tolerance, indicating that the tolerance effect is localized to the aa 170-426 C-terminal peptide region. The Cd responses of transgenic Arabidopsis plants constitutively expressing OsDEP1, OsDEP1(1-169) or OsDEP1(170-426), were similar to the observations in yeast cells, with OsDEP1 and OsDEP1(170-426) transgenic plants displaying Cd tolerance but OsDEP1(1-169) plants showing no such tolerance. In addition, a positive correlation between the transcript levels of OsDEP1 or OsDEP1(170-426) in the transgenics and the Cd content of these plants upon Cd application was observed. As several Arabidopsis loss-of-function heterotrimeric G protein ß and γ subunit gene mutants did not show differences in their Cd sensitivity compared with wild-type plants, we propose that the Cys-rich region of OsDEP1 may function directly as a trap for Cd ions.

Diversity in the complexity of phosphate starvation transcriptomes among rice cultivars based on RNA-Seq profiles.

Rice has developed several morphological and physiological strategies to adapt to phosphate starvation in the soil. In order to elucidate the molecular basis of response to phosphate starvation, we performed mRNA sequencing of 4 rice cultivars with variation in growth response to Pi starvation as indicated by the shoot/root dry weight ratio. Approximately 254 million sequence reads were mapped onto the IRGSP-1.0 reference rice genome sequence and an average of about 5,000 transcripts from each cultivar were found to be responsive under phosphate starvation. Comparative analysis of the RNA-Seq profiles of the 4 cultivars revealed similarities as well as distinct differences in expression of these responsive transcripts. We elucidated a set of core responsive transcripts including annotated and unannotated transcripts commonly expressed in the 4 cultivars but with different levels of expression. De novo assembly of unmapped reads to the Nipponbare genome generated a set of sequence contigs representing potential new transcripts that may be involved in tolerance to phosphate starvation. This study can be used for identification of genes and gene networks associated with environmental stress and the development of novel strategies for improving tolerance to phosphate starvation in rice and other cereal crops.

Ion-beam irradiation, gene identification, and marker-assisted breeding in the development of low-cadmium rice.

Rice (Oryza sativa L.) grain is a major dietary source of cadmium (Cd), which is toxic to humans, but no practical technique exists to substantially reduce Cd contamination. Carbon ion-beam irradiation produced three rice mutants with <0.05 mg Cd⋅kg(-1) in the grain compared with a mean of 1.73 mg Cd⋅kg(-1) in the parent, Koshihikari. We identified the gene responsible for reduced Cd uptake and developed a strategy for marker-assisted selection of low-Cd cultivars. Sequence analysis revealed that these mutants have different mutations of the same gene (OsNRAMP5), which encodes a natural resistance-associated macrophage protein. Functional analysis revealed that the defective transporter protein encoded by the mutant osnramp5 greatly decreases Cd uptake by roots, resulting in decreased Cd in the straw and grain. In addition, we developed DNA markers to facilitate marker-assisted selection of cultivars carrying osnramp5. When grown in Cd-contaminated paddy fields, the mutants have nearly undetectable Cd in their grains and exhibit no agriculturally or economically adverse traits. Because mutants produced by ion-beam radiation are not transgenic plants, they are likely to be accepted by consumers and thus represent a practical choice for rice production worldwide.

Real-time imaging and analysis of differences in cadmium dynamics in rice cultivars (Oryza sativa) using positron-emitting 107Cd tracer.

BACKGROUND: Rice is a major source of dietary intake of cadmium (Cd) for populations that consume rice as a staple food. Understanding how Cd is transported into grains through the whole plant body is necessary for reducing rice Cd concentrations to the lowest levels possible, to reduce the associated health risks. In this study, we have visualized and quantitatively analysed the real-time Cd dynamics from roots to grains in typical rice cultivars that differed in grain Cd concentrations. We used positron-emitting 107Cd tracer and an innovative imaging technique, the positron-emitting tracer imaging system (PETIS). In particular, a new method for direct and real-time visualization of the Cd uptake by the roots in the culture was first realized in this work. RESULTS: Imaging and quantitative analyses revealed the different patterns in time-varying curves of Cd amounts in the roots of rice cultivars tested. Three low-Cd accumulating cultivars (japonica type) showed rapid saturation curves, whereas three high-Cd accumulating cultivars (indica type) were characterized by curves with a peak within 30 min after 107Cd supplementation, and a subsequent steep decrease resulting in maintenance of lower Cd concentrations in their roots. This difference in Cd dynamics may be attributable to OsHMA3 transporter protein, which was recently shown to be involved in Cd storage in root vacuoles and not functional in the high-Cd accumulating cultivars. Moreover, the PETIS analyses revealed that the high-Cd accumulating cultivars were characterized by rapid and abundant Cd transfer to the shoots from the roots, a faster transport velocity of Cd to the panicles, and Cd accumulation at high levels in their panicles, passing through the nodal portions of the stems where the highest Cd intensities were observed. CONCLUSIONS: This is the first successful visualization and quantification of the differences in whole-body Cd transport from the roots to the grains of intact plants within rice cultivars that differ in grain Cd concentrations, by using PETIS, a real-time imaging method.

A major quantitative trait locus for increasing cadmium-specific concentration in rice grain is located on the short arm of chromosome 7.

Large phenotypic variations in the cadmium (Cd) concentration of rice grains and shoots have been observed. However, the genetic control of Cd accumulation remains poorly understood. Quantitative trait loci (QTLs) determining the grain Cd concentration of rice grown in a Cd-polluted paddy field were identified. Using a mapping population consisting of 85 backcross inbred lines derived from a cross between the low-Cd-accumulating cultivar Sasanishiki (japonica) and high-Cd-accumulating cultivar Habataki (indica), two QTLs for increasing grain Cd concentration were found on chromosomes 2 and 7. A major-effect QTL, qGCd7 (QTL for grain Cd on chromosome 7), was detected on the short arm of chromosome 7. It accounted for 35.5% of all phenotypic variance in backcross inbred lines. qGCd7 was not genetically related to any QTLs for concentrations of essential trace metals (Cu, Fe, Mn, and Zn) or those for agronomic traits such as heading date, suggesting that this QTL is specific to Cd. Furthermore, the existence of qGCd7 was confirmed using chromosome segment substitution lines (CSSLs) and an F(2) population from a cross between the target CSSL and Sasanishiki grown in a Cd-polluted paddy soil. To our knowledge, qGCd7 is a novel QTL with major effects for increasing grain Cd concentrations.

A novel plant cysteine-rich peptide family conferring cadmium tolerance to yeast and plants.

We have identified a novel cDNA clone, termed DcCDT1, from Digitaria ciliaris, that confers cadmium (Cd)-tolerance to yeast (Saccharomyces cerevisiae). The gene encodes a predicted peptide of 55 amino acid residues of which 15 (27.3%) are cysteine residues. We found that monocotyledonous plants possess multiple DcCDT1 homologues, for example rice contains five DcCDT1 homologues (designated OsCDT1~5), whereas dicotyledonous plants, including Arabidopsis thaliana, Brassica rapa, poplar (Populus tremula x Populus alba) and Picea sitchensis, appear to possess only a single homologue. GFP fusion experiments demonstrate that DcCDT1 and OsCDT1 are targeted to both the plant cytoplasmic membranes and cell walls. Constitutive expression of DcCDT1 or OsCDT1 confers Cd-tolerance to transgenic A. thaliana plants by lowering the accumulation of Cd in the cells. The functions of the DcCDT1 family members are discussed in the light of these findings.

Root-to-shoot Cd translocation via the xylem is the major process determining shoot and grain cadmium accumulation in rice.

Physiological properties involved in divergent cadmium (Cd) accumulation among rice genotypes were characterized using the indica cultivar 'Habataki' (high Cd in grains) and the japonica cultivar 'Sasanishiki' (low Cd in grains). Time-dependence and concentration-dependence of symplastic Cd absorption in roots were revealed not to be responsible for the different Cd accumulation between the two cultivars because root Cd uptake was not greater in the Cd-accumulating cultivar 'Habataki' compared with 'Sasanishiki'. On the other hand, rapid and greater root-to-shoot Cd translocation was observed in 'Habataki', which could be mediated by higher abilities in xylem loading of Cd and transpiration rate as a driving force. To verify whether different abilities in xylem-mediated shoot-to-root translocation generally account for the genotypic variation in shoot Cd accumulation in rice, the world rice core collection, consisting of 69 accessions which covers the genetic diversity of almost 32,000 accessions of cultivated rice, was used. The results showed strong correlation between Cd levels in xylem sap and shoots and grains among the 69 rice accessions. Overall, the results presented in this study revealed that the root-to-shoot Cd translocation via the xylem is the major and common physiological process determining the Cd accumulation level in shoots and grains of rice plants.